5, a trap in an expression

The trap of a string;

The first time a string is used directly, the JVM caches it into a string pool;

Examples of memory leaks:

String str= "Hello"

String str=str+ "Java"

The sequence of characters contained in a string cannot be changed. So the second STR is pointing to another string object, at which time the Hello string is in memory, the garbage collection mechanism does not recycle it, and then a memory leak occurs.

If you want to use string mutable strings generally use StringBuffer and StringBuilder, but most of the methods in StringBuffer use the synchronized modifier to ensure thread safety, but it can reduce the efficiency of the method to perform. Use the StringBuilder class to represent strings in environments that do not have multiple threads.

Automatic promotion rules for expression types;

1, all byte,short. and char types are promoted to type int.

char (Byte,short) _int_long_float_double

2, the data type of the entire arithmetic expression is automatically promoted to the same type as the highest-level operand of the expression.

Short svalue=5;//defines a short type variable, and the svalue is automatically promoted to int. svalue=svalue-2;//An error occurred when assigning an int type to a short type variable. Svalue-=2 can compile normally, because he is equivalent to svalue= (svalue) (sValue-2); It contains the hermit type conversion. (Almost all binocular operators contain hermit conversions)

int val=3; int result=20/val; System.out.println ("" +result);//output 6, the visible result is the int type.

System.out.println ("Hello" + ' a ' +7); System.out.println (' A ' +7+ "hello"); output Helloa7 104hello when the base type and string type are connected, the system automatically converts the value of the base type to the string type.

2, IME may also cause errors.

Java programs typically cannot contain full-width characters, such as Chinese Spaces, tab tabs.

5, a trap in an expression